Semiempirical calculation of force constants of stretching vibrations in excited states of retinal and its related compounds

The excited states of π-electrons in hexaene, retinal, retinal Schiff-base or protonated retinal Schiff-base are considered by our LCAO-ASMO-SCF-CI methods, under the assumption that the polyene chain of the molecule has all-trans form. And as for each excited state, the total energy of the whole system consisting of π-electrons, atomic cores and σ-bonds is calculated as a function of the terminal bond length, by fixing the other bond lengths at their equilibrium values. On this basis, the relation between force constant of stretching vibration and π-electron bond-order in each excited state is semiempirically determined, according to such parametrization for the energy of σ-bonds that can give a good account of the force constants of stretching vibrations used for the normal-coordinate analysis of retinal.